The invention is directed to the field of deployable platforms and, more particularly, to a precision deployable boom assembly for celestial and terrestrial applications.
Deployable platforms are known for use on spacecrafts such as satellites. These platforms are utilized to support devices such as antennas, cameras, telescopes and various other scientific instruments.
Most deployable platforms are static and capable of only one-time deployment. These platforms are typically unfolded to the deployed position and cannot be retracted once deployed. Some platforms can be retracted. Neither the non-retractable nor the retractable platforms are stable at intermediate positions.
Known platforms are inadequate for applications that require a variable length platform. For example, to measure distant planets and solar systems, instruments can be mounted to a platform and the spacecraft rotated to map a full 360.degree. C. revolution. For different aperture sizes, the distance between the spacecraft and the instruments needs to be varied. This variable positioning is not possible using known static platforms, which provide only a fixed platform length. Known platforms are also incapable of repeatedly positioning the boom at selected intermediate positions between retracted (stowed) and fully deployed with a high degree of accuracy and stability.
It is also necessary in some applications that the platform be extremely dimensionally and positionally stable. High stability is needed for making precise measurements or performing precise mapping using an instrument mounted to the platform. The stability of the platform can be significantly affected by temperature changes. In space, large temperature changes can occur, for example, during travel of satellites between the dark side and the sun side of their orbits. If the platform length is sensitive to temperature changes caused by thermal expansion or contraction, then the dimensional changes in the platform caused by the temperature change can alter the position of the instrument and prevent the instrument from providing precise measurement. Consequently, the mission is degraded at great expense.
The dimensional and positional stability of the platform can also be affected by "deadband" in the platform. Deadband is characterized as a lack of tightness in the platform due to excessive play between the platform members. Deadband causes the platform position to vary, significantly if there is a high amount of deadband in the platform. Consequently, the exact location of instruments mounted on platforms having deadband is indeterminate. In addition, deadband can interfere with the spacecraft control system and prevent the control system from correctly controlling motion of the spacecraft. The problem of spacecraft control is exacerbated in heavy booms having deadband.
Hysteresis in the platform can also affect positional stability. Hysteresis can be caused by the release of frictional loads in the platform, and by the elasticity of the platform members. A high stiffness is desired in the platform to reduce hysteresis and stabilize the platform length.
Another important characteristic of platforms is the natural structural frequency. The structural frequency can be represented by the frequency of lateral movement of the boom. If the structural frequency of the platform is too low, it can couple in with sensors that determine the attitude of the spacecraft and provide signals to thrusters or reaction wheels that control motion of the spacecraft. These sensors have a feedback loop frequency. If the structural frequency of the platform is too close to the feedback loop frequency, then it is not possible to determine whether the motion of the spacecraft is being controlled, or whether the thrusters or reaction wheels are actually just responding to vibrations of the platform and wasting precious fuel. Accordingly, it is desirable that the structural frequency of the platform differ by a safety margin from the feedback loop frequency of the sensors.
It is also important that a stiff platform be achieved without making the platform overly heavy. For spacecraft applications, the weight of the platform must be controlled for launch purposes and to minimize fuel consumption.
Thus, there is a need for a deployable boom assembly that (i) can be adjustably, repeatedly positioned at intermediate locations between retracted and fully deployed; (ii) is dimensionally stable despite changes in temperature; (iii) is positionally stable at all deployed positions; (iv) has a structural frequency that does not couple in with that of sensors that control the motion of spacecrafts; and (v) has a simple construction and is lightweight.